Britain has led international calls for United Nations inspectors in Syria to be allowed to investigate claims that chemical weapons have been used to kill hundreds of people.
Opposition forces have accused government forces loyal to President Bashar al Assad of gassing men, women and children as they slept with the release of deadly fumes over rebel-held suburbs of the capital Damascus.
ITV News Middle East Correspondent Geraint Vincent reports on the shocking attack claims:
Activists variously cited death tolls ranging from about 500 to one account's claim of 1,300 fatalities after shells and rockets reportedly fell around 3:00am local time.
The claims prompted an emergency meeting tonight of the UN Security Council that resulted in a request for inspectors to be given "unrestricted" access to the site of the latest alleged attack.
The inspection team is already in Damascus to probe previous claims of chemical weapons use by Assad forces.
ITV News Europe Correspondent Emma Murphy reports on the diplomatic pressure:
Britain and France earlier joined the United States, the European Union and the Arab League in demanding the move, with Foreign Secretary William Hague saying:
There is no reason not to be given access when (the site) is not so many miles from where they are doing their work now.
Images from Damascus showed dozens of bodies - including small children - laid on the floor of a clinic with no visible signs of injuries and some with foam around their mouths.
The apparent authenticity of the pictures appeared to undermine a response from Russia's foreign ministry that the attack allegations looked like a rebel "provocation" to discredit President Assad.
Foreign Secretary Hague appeared confident that the claims of a massacre were authentic, saying:
I hope this will wake up some who have supported the Assad regime to realise its murderous and barbaric nature.
The Security Council, where Russia has vetoed previous Western efforts to impose UN penalties on Assad, is not expected to take decisive action, though.
Big powers remain at odds and calling for clarity over opposition claims that if proven, would confirm the world's most lethal chemical weapons attack since the 1980s.
In 1988, 3,000 to 5,000 Iraqi Kurds were gassed by Saddam Hussein's forces at Halabja.